A known example of a conventional ink channel structure in an ink jet printer which is a liquid discharge device is that disclosed in FIG. 4 of Japanese Unexamined Patent Application Publication No. 2003-136737.
Specifically, the above Japanese Unexamined Patent Application Publication No. 2003-136737 discloses an arrangement wherein an ink channel is formed of a channel plate such so as to communicate with an ink pressurization chamber.
With the above configuration, the entrance portion of the ink pressurization chamber is formed such that each ink pressurization chamber has its own channel. Also, the ink channel forms a common channel for supplying ink to each of the individual channels for all of the ink pressurization chambers.
FIG. 17 is a diagram schematically illustrating the individual channels and common channel, and the ink liquid chamber (synonymous with the ink pressurization chamber in the above Japanese Unexamined Patent Application Publication No. 2003-136737), describing the actions at the time of discharging ink (arrows in the drawing indicate the movement of ink) in time sequence. In FIG. 17, an ink liquid chamber a, individual channel b, and common channel c are arranged communicably, formed such that the ink can flow (be supplied) from the common channel c→individual channel b→ink liquid chamber.
Further, provided within the ink liquid chamber a is a heating element d, for discharging ink within the ink liquid chamber. In the event that the heating element d is provided on the base of the ink liquid chamber a, normally, a nozzle e is situated on the upper face of the ink liquid chamber a, but in FIG. 17, the nozzle e is illustrated to the right side of the ink liquid chamber a, for the sake of simplifying of the drawing.
First, in the “(1) stationary” state in the drawing, the ink liquid chamber a is filled with ink.
At the time of discharging ink, i.e., in the “(2) expansion” state, the heating element d is rapidly heated, generating a bubble within the ink liquid chamber a. Generating this bubble gives the ink within the ink liquid chamber a flying power, and a part of the ink within the ink liquid chamber a is discharged from the nozzle e as an ink droplet due to the flying power.
Immediately following the above “(2) expansion” state, heating of the heating element d ends. Also, the bubble within the ink liquid chamber a dissipates upon the ink droplet being discharged, so transition is made to the next “(3) contraction”, where the inside of the ink liquid chamber a is depressurized. Further, in the following “(4) replenishing” state, ink of an amount equivalent to that of the discharged ink droplet is replenished to the ink liquid chamber a via the common channel c and the individual channel b.
As described above, the actions of a stationary state, and then expansion, contraction, and replenishing, are repeated when discharging ink.
Now, while gasoline engines, for example, use intake and exhaust valves synchronized with the rotations of the engine, with internal combustion occurring in a state wherein both valves are completely closed, the ink jet printer head shown in FIG. 17 has nothing equivalent to the valves of a gasoline engine.
Accordingly, in order to cause the energy applied to the heating element d to efficiently discharge ink droplets, there is need for expansion of ink to occur in the direction of the nozzle e (toward the right in FIG. 17) as much as possible. In other words, reducing the amount of ink escaping to the individual channel b side (toward the left in FIG. 17) opposite to the nozzle e side at the time of expansion as much as possible will improve the discharge efficiency.
However, with the above-described related art, there is the problem that at the time of expansion by heating the heating element d, a shock wave due to the pressurization is propagated from within the ink liquid chamber a to the individual channel b, and further on to the common channel c side. Also, there is the problem that at the time of contraction, a shock wave due to depressurization is generated through the individual channel b.
FIG. 18 is a diagram illustrating the mutual interference states of shock waves in the stationary, expansion, contraction, and replenishing states shown in FIG. 17.
As shown in FIG. 18, at the time of expansion, a pressurization shock wave occurs at the individual channel b side from the ink liquid chamber a, in addition to that in the discharge direction of the nozzle e. Also, at the time of contraction, a depressurization shock wave due to retraction of the ink to the ink liquid chamber a side from the individual channel b side occurs. It is estimated that these pressurization shock wave and depressurization shock wave affect even the common channel c. Such shock waves affect ink liquid chambers a adjacent to the ink liquid chamber a which has performed the discharging action. For example, in the event that a pressurization shock wave reaches an adjacent ink liquid chamber a, the pressure within that ink liquid chamber a increases. Also, in the event that a depressurization shock wave reaches an adjacent ink liquid chamber a, the pressure within that ink liquid chamber a decreases.
FIG. 19 is a diagram for describing the relation between the pressure within the ink liquid chamber a and the discharged ink droplet. FIG. 19 illustrates, from top down in sequential order, the states of when stationary, when generating a bubble, when the bubble is dissipating, and when the ink droplet is being discharged. Also, in the drawing, the left-side column (A-1) indicates a case wherein the pressure within the ink liquid chamber a is smaller than a suitable value (pressure<suitable value), the middle column (A-2) indicates a case wherein the pressure within the ink liquid chamber a is at the suitable value (pressure=suitable value), and the right-side column (A-3) indicates a case wherein the pressure within the ink liquid chamber a is greater than the suitable value (pressure>suitable value).
As shown in FIG. 19, in the event that the pressure within the ink liquid chamber a is at the suitable value, the meniscus of the ink droplet prior to discharge (when stationary) is concaved as to the discharging face of the nozzle e, with the pressure within the ink liquid chamber a being balanced against the surface tension acting upon the nozzle edge and the external air pressure, thereby maintaining a suitable position.
In the event that the pressure within the ink liquid chamber a changes, the amount of ink within the ink liquid chamber a changes accordingly, so the amount of the ink droplet discharge changes. That is to say, in the event that the pressure within the ink liquid chamber a is low, the amount of the ink droplet discharge is smaller, as shown in the left-side column (A-1) in the drawing. On the other hand, in the event that the pressure within the ink liquid chamber a is high, the amount of the ink droplet discharge is greater, as shown in the right-side column (A-3) in the drawing.
Changes in the amount of the ink droplet discharged changes in this way are manifested in the results of the ink droplets landing as change in ink density (density irregularities).
FIG. 20 is a graph representation of the results of performing ink droplet discharge with an ink jet printer line head manufactured for 600 dpi, and measuring change in density of the discharged ink as change (volume/weight) in the ink droplets. In the drawing, the horizontal axis represents the nozzle position, and the vertical axis represents the density (the higher in this drawing, the darker the color). With this example, portions where one dot is recorded for each pixel over 32 nozzles, and portions where no ink droplets are discharged (blank; white areas), are arrayed in an alternating manner.
Also, the upper part of FIG. 21 illustrates how the change in density appears for the portion in FIG. 20 surrounded by the single-dot broken line in the form of contrasting densities, by showing lightness information alone. Also, the lower part of FIG. 21 illustrates an ideal state wherein no change due to pressure fluctuation occurs, as a reference value, at the average density value (160) of the upper half.
The data shown in FIG. 20 and the diagram at the upper part of FIG. 21 do not represent instantaneous changes which actually occur, and were created by averaging data of each nozzle e recording over a certain length (actually, discharge was performed once per pixel, over a length of 196 pixels which is approximately 25 mm, by discharging 196 times).
It can be understood from this diagram that, regardless of averaging over a long period, the property of each nozzle e does not stay around the density of 160 but rather fluctuates widely, i.e., that standing waves are present. Further, the fact that such visible fluctuations remain even for average values can be thought to mean that even greater fluctuations occur on an instantaneous level.
An example of a conceivable method to suppress manifestation of density irregularities due to the effects of shock waves occurring at the time of discharging ink droplets and the air bubble contracting as described above, is to, firstly, make the individual flow channel b narrower (make the cross-sectional area of the channel smaller), or secondly, not make the individual flow channel b narrower but longer.
These methods can reduce interference among the ink liquid chambers due to discharge, thereby reducing irregularities in the amount of ink droplets discharged therefrom.
However, the above methods have problems in that the time for replenishing (refilling) the ink liquid chamber a with ink following discharge of the ink droplet takes longer due to increased channel resistance of the individual channel b. Also, making the individual channel b narrower means that undesired matter, dust, and the like, can become stuck therein just that much more readily, incapacitating ink discharge. Further, the above second method (method of forming the individual channel b longer) has the problem that the head increases in size.
Accordingly, the problems to be solved by the present invention is to reduce the effects of shock waves and to reduce the difference in density among the discharged ink droplets, without extending the refill time, without increasing the risk of faulty discharge due to undesired matter and dust and the like, and without increasing the size of the head.